Say I want to test the waters for selfhosting email, and I already have my how domains setup with SaaS like Google workspace and equivalent. Is there a way to setup mx records so that both google and my own server gets email for a while? This would be useful to test the waters over a few months before fully migrating
Not with MX but, look at google's split domain documentation. You can either have them handle the domain and forward you a copy, or you can have your own domain be the primary and forward to google. I have been using the latter for a few years now since not all of the users in the domain are using Google Workspace. They have a special address for forwarding to so you don't get into a loop. It has been working flawlessly for us.
You can set up a lower-priority MX to point to Google, so if your server fails, then email is delivered to Google. But if your server is misconfigured and returns permanent 5xx errors for legitimate emails, then it won't work, and the emails won't be delivered to Google.
No easy answer here. Individual MTAs or a cluster of them typically live under one unique domain. In your scenario, you'd have to point your existing records (or just MX) to your self-hosted instance, and have your self-hosted instance relay/autoforward to Gmail under a different domain. This might entail simply setting your Gmail back to @gmail.com.
Not really, SMTP relays will only send messages once, to one server.
But it’s not receiving that is the problem, that is generally fine, if ports are open at ISP / network level. It is the sending that is often tricky. Sending email on the other hand can be done from multiple servers (if SPF correctly configured) And nothing prevents you from sending email directly from your own relay. You could try that, and reception would not be affected.
